Abundant and Dynamically Expressed Mirnas, Pirnas, and Other Small Rnas in the Vertebrate Xenopus Tropicalis
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Downloaded from genome.cshlp.org on September 26, 2021 - Published by Cold Spring Harbor Laboratory Press Letter Abundant and dynamically expressed miRNAs, piRNAs, and other small RNAs in the vertebrate Xenopus tropicalis Javier Armisen,1,2,3 Michael J. Gilchrist,1,3 Anna Wilczynska,2 Nancy Standart,2 and Eric A. Miska1,2,4 1Wellcome Trust Cancer Research UK Gurdon Institute, University of Cambridge, The Henry Wellcome Building of Cancer and Developmental Biology, Cambridge CB2 1QN, United Kingdom; 2Department of Biochemistry, University of Cambridge, Cambridge CB2 1GA, United Kingdom Small regulatory RNAs have recently emerged as key regulators of eukaryotic gene expression. Here we used high- throughput sequencing to determine small RNA populations in the germline and soma of the African clawed frog Xenopus tropicalis. We identified a number of miRNAs that were expressed in the female germline. miRNA expression profiling revealed that miR-202-5p is an oocyte-enriched miRNA. We identified two novel miRNAs that were expressed in the soma. In addition, we sequenced large numbers of Piwi-associated RNAs (piRNAs) and other endogenous small RNAs, likely representing endogenous siRNAs (endo-siRNAs). Of these, only piRNAs were restricted to the germline, suggesting that endo-siRNAs are an abundant class of small RNAs in the vertebrate soma. In the germline, both endogenous small RNAs and piRNAs mapped to many high copy number loci. Furthermore, endogenous small RNAs mapped to the same specific subsets of repetitive elements in both the soma and the germline, suggesting that these RNAs might act to silence repetitive elements in both compartments. Data presented here suggest a conserved role for miRNAs in the vertebrate germline. Furthermore, this study provides a basis for the functional analysis of small regulatory RNAs in an important vertebrate model system. [Supplemental material is available online at http://www.genome.org.Short read sequence data from this study have been submitted to NCBI Gene Expression Omnibus (GEO) (http://www.ncbi.nlm.nih.gov/geo/) under series accession no. GSE14952.] Short RNAs have recently emerged as abundant regulators of gene Lee 2004), Drosophila (Czech et al. 2008; Ghildiyal et al. 2008; expression in many eukaryotes, including plants, animals, and Kawamura et al. 2008; Okamura et al. 2008), and mouse oocytes fungi (Sharp 2009). The lin-4 and let-7 miRNAs were the first type (Tam et al. 2008; Watanabe et al. 2008). endo-siRNAs are enriched of endogenous short regulatory RNAs to be identified in eukaryotes in the germline of animals and map to various genomic loci (Lee et al. 1993; Reinhart et al. 2000); since then many functional including repetitive elements, pseudogenes, palindromes, and small RNAs have been identified in organisms as diverse as regions where both strands are transcribed. Like miRNAs, endo- roundworms, flies, fish, frogs, mammals, flowering plants, mosses, siRNAs interact with Argonaute proteins. endo-siRNAs likely anemones, sponges, and even viruses, using genetics, molecular have roles in silencing of transposable elements or pseudogenes cloning, and predictions from bioinformatics (Lagos-Quintana (Okamura et al. 2008). et al. 2001; Lau et al. 2001; Lee and Ambros 2001; Llave et al. 2002; A third class of 25–30 nt RNAs has been identified in Dro- Reinhart et al. 2002; Lim et al. 2003; Pfeffer et al. 2004; Arazi et al. sophila, zebrafish, mice, rats, anemones, and sponges and has been 2005; Axtell and Bartel 2005; Watanabe et al. 2005; Grimson et al. named Piwi-associated RNAs or piRNAs (Grimson et al. 2008; 2008). In cells, miRNAs are tightly bound by proteins of the Ago Klattenhoff and Theurkauf 2008). By definition piRNAs interact clade of the Argonaute superfamily of RNA-binding proteins with proteins of the Piwi clade of the Argonaute superfamily. (Cerutti et al. 2000). miRNAs are thought to inhibit efficient piRNA populations are complex; there are hundreds of thousands translation of target mRNAs or to control mRNA decay. of unique piRNAs in mammals. Piwi and piRNAs are required for Another class of small RNAs, 21–24 nucleotides (nt) endoge- transposon silencing: for example, in Drosophila the piRNAs of the nous siRNAs, was first discovered in plants in response to viral flamenco locus control the gypsy retrotransposon (Desset et al. infection (Hamilton and Baulcombe 1999; Llave et al. 2002). These 2003; Brennecke et al. 2007). The piRNAs of C. elegans are unique RNAs are thought to represent endogenous instances of short in- in that they are 21 nt short RNAs with distinct genomic organi- terfering RNAs (siRNAs), the mediators of RNAi (Fire et al. 1998; zation and biogenesis, but a conserved role in transposon silencing Tuschl et al. 1999; Zamore et al. 2000). More recently, endo-siRNAs (Ruby et al. 2006; Batista et al. 2008; Das et al. 2008; Wang and have also been identified in Caenorhabditis elegans (Ambros and Reinke 2008). Previously, small RNAs have also been grouped together based on their genomic location as repeat-associated small RNAs 3These authors contributed equally to this work. (rasiRNAs) in plants, fungi, Drosophila, and zebrafish (Llave et al. 4 Corresponding author. 2002; Reinhart et al. 2002; Aravin et al. 2003; Chen et al. 2005b). E-mail [email protected]; fax 44-1223-767225. Article published online before print. Article and publication date are at These can now be reclassified as endo-siRNAs or piRNAs based http://www.genome.org/cgi/doi/10.1101/gr.093054.109. on their size, biogenesis, and associated Argonaute superfamily 1766 Genome Research 19:1766–1775 Ó 2009 by Cold Spring Harbor Laboratory Press; ISSN 1088-9051/09; www.genome.org www.genome.org Downloaded from genome.cshlp.org on September 26, 2021 - Published by Cold Spring Harbor Laboratory Press Xenopus small RNAs proteins (Okamura et al. 2008; Malone and Hannon 2009). Al- though endo-siRNA and piRNA pathways are distinct, in ani- mals, endo-siRNAs and piRNAs are 29O-methylated at the 39 end (Horwich et al. 2007; Tam et al. 2008; Watanabe et al. 2008). While the function of this modification remains unclear in animals, in plants, 29O-methylation stabilizes miRNAs and endo-siRNAs (Yang et al. 2006). Xenopus laevis has been used widely as a model system for the study of oocyte development and maturation, including the regulation of gene expression at the level of translation and RNA localization. Xenopus oogenesis is subdivided into six stages (I–VI) based on features such as diameter, pigmentation color, and the amount of yolk in the cytoplasm. Stage VI oocytes are arrested in first meiotic prophase, and can be matured into eggs, arrested in MII metaphase, by progesterone. While previous work in Xenopus has identified a number of miRNAs through cloning and comparative genomic approaches, little is know about small RNAs population during Xenopus oogenesis. Microarrays, North- ern blotting, and in situ hybridization have been used to deter- mine miRNA expression during embryogenesis and adult frog (Watanabe et al. 2005; Hikosaka et al. 2007; Michalak and Malone 2008; Tang and Maxwell 2008; Walker and Harland 2008). How- ever, recent advances in sequencing technology have allowed the more complete assessment of small RNA species in animals, plants, and fungi. Here we applied Illumina sequencing (formerly known as Solexa sequencing) to determine the expression of small RNAs in the Xenopus tropicalis germline and somatic tissues. This work represents the first example of small RNA high-throughput se- quencing in an amphibian. Using this approach we identify abundant populations of miRNAs, piRNAs, and other small RNAs in the germline and soma of X. tropicalis. We hope that these data might set the stage for the biochemical analysis of small RNA pathways in a powerful model system, the Xenopus oocyte. Results Figure 1. Expression of small RNAs and core protein components in X. tropicalis.(A) Total RNA was isolated from X. tropicalis adult liver and stage The Xenopus female germline expresses different classes I and stage II oocytes. RNA was size-selected using the miRVana kit. Ten of small RNAs micrograms of this RNA was subjected to b-elimination or not as indicated and analyzed on a denaturing gel after 59 end-labeling. Arrows indicate We first isolated total RNA from oocytes of Xenopus tropicalis and RNA that likely represent miRNAs, siRNAs, and piRNAs. (B,C ) The ex- Xenopus laevis at different stages of oogenesis. Small RNAs were pression of core components of small RNA pathways in Xenopus oocytes, eggs, and somatic tissues was assayed using RT-PCR and qRT-PCR. For size-selected and resolved on polyacrylamide gels (Supplemental each experiment equivalent oocyte total RNA was reverse transcribed. E, Fig. S1). We observed a strikingly similar pattern of small RNAs in egg; L, liver; I, intestine; N.D., not done. (B) RT-PCR experiment. Control the different oogenic stages in X. tropicalis and X. laevis, where experiment without the addition of reverse transcriptase. RT-PCR primers short ;22, ;24, and ;30 nt RNAs were most abundant. Given are listed in Supplemental material, Armisen_SupData4.xls. (C ) qRT-PCR for the four Piwi-related genes described in X. tropicalis. the known sizes of microRNAs (miRNAs), endogenous siRNAs (endo-siRNAs), and Piwi-interacting RNAs (piRNAs) in other spe- cies we suspected that these bands could represent miRNAs, endo- siRNAs, and piRNAs in X. tropicalis and X. laevis, respectively. To contrast, in RNA from stage I and stage II oocytes, two small test this hypothesis, we took advantage of the fact that in mouse RNA bands did not shift mobility after b-elimination, which was and Drosophila melanogaster endo-siRNAs and piRNAs are 29O- consistent with 29O-methylation of the 39-most nucleotide. Based methyl-modified at the 39-most nucleotide, while miRNAs are on these observations we concluded that the most abun- not (Horwich et al.